Note
from Remsen: Below is the proposal submitted to, passed by, and adopted
by NACC; see latest NACC Supplement in Auk 2018).I made a few minor edits for SACC.For NACC members’ comments on this
proposal, see: http://checklist.aou.org/nacc/proposals/comments/2017_A_comments_web.html, proposal
2017-A-8).Northern Harrier (currently
C. cyaneus in our classification) is
a wintering (NB) species in the Colombia and Venezuela)

Treat New World Circus
(c.) hudsonius as a separate species from Old World Circus cyaneus

Description of the problem: In 2015, NACC considered but
narrowly rejected (by seven votes to five) a proposal (2015-C-9) to split Circus cyaneus and C. hudsonius, the latter currently considered a subspecies of the
former. (See: http://checklist.aou.org/nacc/proposals/prior_2015.html,
2015-C-9) The primary basis behind some committee members’ rejection of the
split was the weak genetic sampling coupled with the relatively low genetic
divergence (1.1 to 1.7%). Another objection was the lack of demonstrated vocal
differences.

New information: Etherington and Mobley (2016)
compared cyaneus and hudsonius in DNA, plumage, measurements,
and ecology, and recommended on these bases that they should be considered
separate species. These authors sequenced a few new samples of cyt b (8 frozen
tissues of hudsonius and 3 toepads of
cyaneus), and used GenBank samples of
COI from bar-coding (4 hudsonius, 7 cyaneus) and Oatley et al.’s (2015) ND1
sequences for further analyses.They found in each of their analyses that cyaneus and hudsonius
form monophyletic clades. They also found genetic distances ranging from 1.3 to
1.8% between the two taxa. They noted that in several other cases genetic
distance between undisputed species-pairs of raptors is in this range, well
below 2%.

The morphological data presented by Etherington and Mobley
(2016) confirm that, sex-for-sex, hudsonius
is larger than cyaneus. They review
the sexual and age-related differences between the taxa, noting that adult
males differ by 13 morphological characters, females by about four, and
juveniles by three or more.

In the discussion, Etherington and Mobley (2016) noted “numerous differences between cyaneus and hudsonius
when it comes to vocalization, habitat, distribution and movements, mate choice
and breeding biology”, and yet their vocal analysis is limited to two
paragraphs summarizing characteristics of sonograms in the Western Palearctic
handbook (Cramp and Simmons 1980) and the BNA account (MacWhirter and Bildstein
1996); no mention is made of online resources nor commercial CDs. They
concluded based on this tiny sample that the taxa differ vocally in that cyaneus gives kek calls at a faster rate than does hudsonius, both in male and female distress calls.

Etherington and Mobley (2016) then compared life-history
information extracted from the literature. They argue that hudsonius is a bird of wetlands, prairies, dry grasslands, and
agricultural areas, whereas cyaneus
breeds in heather moorland, sand dunes, young coniferous forest, sedge-rich
northern lakes, and woodland (both open- and closed-canopy). (Note that Old
World marsh habitat is typically occupied by the larger marsh harriers.)
Although cyaneus breeds at least
mostly in dry upland habitats, hudsonius
typically breeds among reed beds in marshes, even constructing platforms that
raise the nest above the water level. Other life-history comparisons given by
these authors include that in cyaneus
females have been recorded as displaying much more than males, whereas the
reverse has been found in hudsonius;
and that female hudsonius have been
recorded as being much more capable of successfully raising young successfully
after desertion by the male than is cyaneus.
Despite the statement quoted in the previous paragraph, data are not presented
on differences in mate choice, and the differences discussed in distribution
and migratory route provide no data relevant to species status.

Subsequent treatments: As far as I am aware, SACC is the
only major relevant avian taxonomic entity that has yet to adopt this split.

Effect on SACC: New World C. c. hudsonius (Northern Harrier) would be considered specifically
distinct but would not necessarily require an English name change.

Recommendations: Etherington and Mobley (2016)
provided further evidence that cyaneus
and hudsonius are discrete lineages
with differing breeding habitats and possibly with behavioral differences. The
vocal differences alluded to therein simply do not hold up, however, I
reexamined the recordings from commercial CDs that led to our conclusion in
Rasmussen and Anderton (2005) that vocalizations are broadly similar (mainly
Roché 1996 and Chappuis 2000) for cyaneus,
and compared these and the few for cyaneus
on xeno-canto with the now-extensive sample for hudsonius from several sources. This expanded sample shows that
there is complete overlap in rate of kek
calls, and I can hear no intertaxon differences in quality of these or the
other main vocalization type, the more prolonged mewing calls. That is not to
say that careful study of homologous display sounds would not turn up
differences, but these must be subtle at best. Because many diurnal raptors
lack obvious vocal differences, especially among those (like harriers) that
tend to be fairly quiet, I do not think this is particularly consequential.
(Note the dearth of recordings of cyaneus
even on xeno-canto, and a total lack thereof on Macaulay Library and IBC sites,
which is surprising for a widely distributed Palearctic species.)

However, these taxa are well-differentiated morphologically,
more so than most other Holarctic-distributed species (e.g. subspecies of
Golden Eagle, Rough-legged Hawk, Common Raven, Greater Scaup, and Common
Goldeneye, for example). I have thought them better treated as separate species
for a couple of decades now, ever since preparing materials for our book. In
retrospect, I think that my quote from Oberholser in the original proposal may
have led some to be swayed by his viewpoint, which was probably based on
examination of specimens with folded wings, and thus not a full accounting of
the prominent differences, especially in adult males.

Comments from Claramunt: “YES.
Morphometrically there’s a lot of overlap and I don’t think there’s anything
“significant” there. Although they don’t show quantitatively that there is
discontinuity in the variation, they say that all individuals of hudsonius can
be distinguished from all individuals of cyaneus based on
multiple plumage traits. The illustrations and photographs that I’ve seen so
far corroborate that. The differences are not outstanding, but they seem
diagnostic. What is missing from this proposal is the results of Oatley et al.
(2015), which showed that hudsonius is more closely related to C.
cinereus than to cyaneus based (mostly) on ND1 mtDNA
sequences (however, some CO1 sequences that I reanalyzed show cyaneus and hudsonius as
sister, so don’t take Oatley et al. 2015 as the final word on this). In any
case, I think that the evidence so far indicates that hudsonius is
a species-level taxon and evidence of reproductive compatibility with cyaneus is
completely absent.”